1. Field of the Invention
This invention relates to electrochemical machining of metals and alloys and more particularly to electrochemical machining of metals and alloys using modulated reverse current.
2. Brief Description of the Prior Art
Hard, high strength metals and alloys have come to be widely used in industry because of their favorable mechanical properties. Such alloys are useful, and even essential, for fabricating mechanical parts that have the mechanical strength and resistance to wear and corrosion that are required in modern industrial practice. Dies for manufacturing metal parts in the automobile and aircraft industries, parts for aircraft and aerospace vehicles, and parts for machine tools are typical uses to which these alloys have been applied. However, the hardness, wear resistance, and passivity of these metals and alloys that make them so useful also cause them to be very difficult and expensive to shape by conventional machining methods.
Accordingly, electrochemical machining (ECM) has come to be used as a method of preparing mechanical parts from such hard, high strength metals and alloys. In ""numerous cases, these hard, high strength metals and alloys are passive, i.e., they form a thin protective surface oxide film. ECM is effective because it generally provides a high rate of metal removal from the workpiece, it lends itself to the generation of complicated contours and profiles, tool wear is absent or minimal, and the machined surfaces are generally free from burrs and/or scratches. Because ECM is a non-mechanical metal removal process, it is applicable to machining of any electrically-conductive material without regard to the mechanical and/or thermal properties of the workpiece, such as hardness, elasticity and thermal conductivity. Consequently, the principles of electrochemical machining have been applied to removal of relatively large amounts of metal to generate structural structural shapes, as well as to electrochemical polishing, used to form smooth surfaces, and to electrochemical deburring, used to remove small protuberances, e.g., flash, burrs, and the like, on metal parts formed by other machining methods such as casting, metal cutting, forging, and the like. All these applications are often discussed under the general heading of electrochemical machining.
Electrochemical machining (ECM) is conducted in an electrolytic cell by applying a positive (anodic) potential to the workpiece and a negative (cathodic) potential to the tool used to shape the workpiece. In conventional ECM processes, continuous direct current is used. The interelectrode gap will vary, depending on the particular application. In conventional electrochemical machining, wherein substantial amounts of metal are removed, the interelectrode gap is relatively small, typically 0.5 mm to about 10 mm (although larger gaps may be used), and is often kept relatively constant by advancing the tool toward the workpiece as the workpiece surface is eroded. The rate of advance evidently is highly dependent on the conditions, such as current, electrolyte and the particular metal being formed, but is typically in the range of 0.1 to about 10 mm per minute. For electrochemical polishing, the gap is generally relatively small, typically 0.1 mm to 1 mm, and the tool is usually fixed with reference to the surface to be polished. For electrochemical deburring, the gap is relatively large, typically 5 mm to 50 mm, or even greater, and the tool is typically fixed. Electrolyte (typically an aqueous solution of NaCl or NaNO3) is supplied to flow through the gap to maintain the electrically conductive path for electrochemical dissolution on the workpiece surface and to carry away the waste products and heat generated by the electric current. In some applications, particularly electrochemical machining and electrochemical polishing, the electrolyte flow rate may be large, typically 10-60 meters per second.
In ECM, the dissolution rate of the metal from the surface of the workpiece, the surface finish, and the precision of the machined piece are related to the kinetics and stoichiometry of the electrode reactions. These parameters are strongly influenced by prevailing mass transport conditions. It is well known that the influence of local variations within the inter-electrode gap on mass transport rates and electrolyte conductivity due to the presence of gas bubbles and the resulting heating can be minimized by working at high electrolyte flow velocity. However, high electrolyte flow rates require an elaborate pumping system and a heavy machine frame to maintain rigidity. The resulting installation and operating costs present a major limitation to the wider application of the ECM process.
Particular difficulties have been encountered by both industrial and laboratory researchers in the conventional ECM of hard passive metals and alloys, such as surface defects and unsatisfactory dimensional accuracy control. Hard passive metals and alloys possess a high resistance to corrosion due to the formation of a thin protective oxide film on their surfaces. Therefore, high voltage or current is needed to break the oxide film during electrochemical machining of such materials. High voltage or current results in more undesired products and heat, which require higher electrolyte flow rates to remove. Additionally, the dimensional accuracy is poor due to 1) non-uniform electrolyte hydrodynamic conditions in the interelectrode gap caused by undesired products and heat, and 2) change in the tool size and shape caused by precipitation of metal hydroxide. Surface quality is also poor due to the cavitation caused by high electrolyte flow rate and gas bubbles leading to non-uniform hydrodynamic conditions in the gap. These problems, which are not, in fact, limited to ECM of passive materials, are discussed in Kozak, J., et al., xe2x80x9cThe Study of Thermal Limitation of Electrochemical Machining Processesxe2x80x9d, Transactions of NAMRI/SME, Vol. 15, pp. 159-164 (1997).
Recent research has focused on pulsed current electrochemical machining (PECM) to machine hard passive alloys, such as nickel-based superalloys and titanium alloys, and to improve dimensional accuracy and surface quality by 1) localizing current, 2) improving electrolytic hydrodynamic uniformity in the interelectrode gap by removing undesired products and heat during off-time, 3) reducing electrode gap, and 4) reducing the electrolyte flow rate. However, PECM also has some problems, especially for hard passive alloys. For the hard passive alloys, if sufficient oxygen is present, the oxide film is self-healing and reforms almost instantaneously, however non-uniformly, during the off-time in PECM. As a result, a partial film breakdown often occurs during the next on-time period, resulting in surface cavitation with pitting and high roughness. Additionally, the dissolved metal from the workpiece may precipitate near the cathode (tool) due to the high pH layer generated during the hydrogen evolution reaction at that electrode. The possible deposit of metal hydroxide on the cathode (tool) will change the tool size and shape. Consequently, dimensional accuracy becomes poor.
Accordingly, a need has continued to exist for a method of electrochemical machining that avoids the difficulties experienced with the procedures of the prior art.
The problems encountered in conventional electrochemical machining (ECM) and pulsed current electrochemical machining (PC ECM) processes have now been alleviated by the method of the invention wherein a modulated reverse electric field is used in the electrochemical machining process (MR-ECM).
Accordingly, it is an object of the invention to provide a method of electrochemical machining.
A further object is to provide a method of electrochemical machining using modulated reverse-field electrolysis.
A further object is to provide a method of electrochemical machining adapted to the shaping of passive metals and alloys.
A further object is to provide a method of electrochemical machining with improved dimensional accuracy.
A further object is to provide a method of electrochemical machining that improves dimensional accuracy by localizing the electric current.
A further object is to provide a method of electrochemical machining that improves hydrodynamic uniformity of the electrolyte within the interelectrode gap.
A further object is to provide a method of electrochemical machining that reduces or prevents deposition of metal hydroxides on the tool.
A further object is to provide a method of electrochemical machining that improves dimensional accuracy by reducing the interelectrode gap.
A further object is to provide a method of electrochemical machining that decreases and controls the pH of the electrolyte near the tool (cathode).
A further object is to provide a method of electrochemical machining that improves the surface quality of the machined workpiece.
A further object is to provide a method of electrochemical machining that reduces cavitation in the interelectrode gap caused by non-uniform hydrodynamic conditions.
A further object is to provide a method of electropolishing surfaces of metal objects.
A further object is to provide a method of electropolishing surfaces of passive metals and alloys.
A further object is to provide a method of electrochemical deburring of metal objects.
A further object is to provide a method of electrochemical deburring of objects made from passive metals and alloys.
A further object is to provide a method for reducing or eliminating the formation of a passive layer on a workpiece in electrochemical machining, electrochemical polishing and electrochemical deburring.
Further objects of the invention will become apparent from the description of the invention which follows.